1. Field of the Invention
The instant disclosure relates to a solar energy device; in particular, to an electricity storage device for a solar energy harvesting device.
2. Description of Related Art
The energy which is existed on earth falls into shortage yearly by yearly. Such as coal; petroleum; ethanol; nuclear energy; water energy; geo-thermal energy; wind energy; and renewable bio-energy etc., all encounter different drawbacks and difficult reasons on cost; safety; green; and productivity. Because of the usage and human population are increased rapidly, it is inevitable to have a clean; reliable; and cost effective energy for human being to utilize it daily. There are two candidates which can meet the previous criteria—nuclear fusion and solar energy. The nuclear fusion is good but it still cannot go to commercialize due to technical barrier, while solar is the very candidate to fulfill the determinate role on energy sources. Although solar energy harvesting is a feasible and reasonable energy source as compared to other existed candidates, there are some other issues to be breakthrough to make it become a mighty energy source. The issues are solar cells efficiency and photo-voltaic energy harvest/transfer efficiency. III-V compound cell with new quantum dot technology shows amazing efficiency over 70% photo-voltaic conversion but it can be only used on special applications due to its extraordinary fabrication cost. Currently commercial solar cell is silicon based with about up to 21% photo-voltaic conversion efficiency. Even there are some other type solar cells, for example organic polymer and II-VI compound are announced but the reliability; durability; and cost make it is unable to be a suitable candidate. Lately most of solar cell manufactures invest more and more on the improvement of silicon-based solar cells with light intensity collection; incident light recycling; multiple-path absorption, etc. So far, there is not a good photo-voltaic transfer design to accommodate the harvested solar energy transfer into stored voltaic energy and/or usable electric energy. Most of design needs to be under high light incidence to trigger the harvesting energy transfer, for example more than 30-50K Lux.
Currently solar energy harvesting is quite limited to storage and driving capability. The battery type of storage and driving has a contradiction between dual-way demands. For storage, the harvesting (charge) speed needs to be moderate to prolong the battery life and to reduce the storage cost, while driving capability is in the opposite demanding. To co-exist with current commercial battery types, for example USB or adaptor charge controlled battery, the direct energy transfer from solar energy harvesting is impossible due to restriction of charge voltage range values. Hence to use a simple harvesting and transfer boost and charge controller is not possible to meet the requirements of low-light solar energy harvesting and electric energy transfer.